1. Field of the Invention
The present invention relates to an ink-jet printhead, and more particularly, to an ink-jet printhead having a fusing-type data input/output capability.
2. Description of the Related Art
In general, ink-jet printheads are devices to print a predetermined color image by ejecting small volumes of droplets of printing ink at desired positions on a recording sheet. These ink-jet printheads are divided by two driving methods according to the ink ejection mechanism. First, ink-jet printheads may use a thermal driving method, which eject ink droplets by the expansion force of bubbles generated in ink by a heat source. Also, ink-jet printheads may use a piezoelectric driving method, which eject ink droplets by the pressure applied to ink due to the deformation of a piezoelectric body.
Hereinafter, the ink ejection mechanism in the thermal ink-jet printheads will be described in greater detail. When current having a pulse shape flows through a heater formed of a resistant heating material, heat is generated in the heater, and ink adjacent to the heater is instantaneously heated to about 300° C. As such, ink is boiled, and bubbles are generated in the ink, expand, and apply pressure to an inside of an ink chamber filled with the ink. As a result, the ink in the vicinity of a nozzle is ejected in a droplet shape through the nozzle from the ink chamber.
Here, the thermal driving method includes a top-shooting method, a side-shooting method, or a back-shooting method according to a growth direction of the bubbles and an ejection direction of the ink droplets.
The top-shooting method is a method in which the growth direction of the bubbles is the same as the ejection direction of the ink droplets. The side-shooting method is a method in which the growth direction of the bubbles is perpendicular to the ejection direction of the ink droplets. The back-shooting method is a method in which the growth direction of the bubbles is opposite to the ejection direction of the ink droplets.
The ink-jet printheads using the thermal driving method should satisfy the following requirement. First, manufacturing of the ink-jet printheads must be simple, costs must be low, and mass production thereof must be possible. Second, in order to obtain a high-quality image, crosstalk between adjacent nozzles must be suppressed and an interval therebetween has to be narrow. That is, in order to increase the number of dots per inch (DPI), a plurality of the nozzles must be arranged with narrow intervals therebetween. Third, in order to perform a high-speed printing operation, a period in which the ink chamber is refilled with ink after being ejected from the ink chamber must be as short as possible, and heated ink must be quickly cooled such that a driving frequency can increase.
Currently, ink-jet printheads have been developing so as to realize high printing resolution and high-speed printing. For this purpose, ink-jet printheads having several hundreds or more of nozzles of small sizes have been developed.
Meanwhile, various driving circuits to drive the nozzles and various digital logic circuits to address the nozzles are being embedded in a printhead chip. As such, various important electrical characteristics inside the head chip must be accurately controlled. These values include resistance of the heater to generate the bubbles in the ink-jet printhead, impedance of a metal-oxide semiconductor field effect transistor (MOS FET) to drive the nozzles and a temperature constant of a temperature sensor. These characteristics have a predetermined range of distribution according to several variables in a semiconductor manufacturing process of the head chip. In order to accurately drive and control several hundreds of the nozzles, the above-mentioned characteristic values are memorized for each head chip, and desired performances can be achieved only when the printhead is driven under optimized conditions in which these characteristic values are considered.
For this purpose, at an initial stage, by attaching an additional electrically erasable and programmable read only memory (EEPROM) to an ink cartridge, the above-mentioned electric characteristic values are recorded, and an identity (ID) code of the head chip or ink retaining quantity in an ink tank are memorized. However, if an additional EEPROM is used, when the head chip is manufactured and finished at a wafer level, it is impossible to measure electrical characteristics and input values thereof during an inspection process. In this case, the above characteristic values have to be input after the cartridge is manufactured. Thus, productivity decreases, and due to additional parts, costs increase.
In order to solve these problems, a read only memory (ROM) to store data is manufactured together in a driving circuit portion when the printhead chip is manufactured. However, the number of additional semiconductor manufacturing processes to implement a ROM circuit increases in a driving MOS circuit of the printhead, thereby increasing costs of the head chip.
Recently, considering that the input capability of electrical data is not large, by using a fusing-type data recording method other than a conventional ROM method, a memory device can be implemented in the head chip without an additional process. As a result, an ink-jet printhead using the thermal driving method, by which ink is sprayed by bubbles generated by heating ink, includes a plurality of heaters to heat the ink, a driving FET array, a digital logic circuit to address each of the heaters, and connection pads. Furthermore, the printhead includes a fuse array which records data such as resistance of the heaters, impedance of the MOS FET, and an ID code of the head chip forms a part of the head chip.
In order to input data into the fuse array, fusing of a fuse member to form the fuse array is necessary. In order to fuse the fuse member with the least energy, it is important to properly select the material, shape, and thickness of the fuse member.
In general, a material used for the fuse member is the same as the material of an electrode formed under a heater layer to eject ink, or as the heater. In order to fuse the fuse member, a predetermined amount of current must flow through the fuse member.
If the material of the fuse member is the same as the material of the electrode, resistance of the fuse member is very small (Rs<0.1Ω/□). Thus, in order to form a resistant body, a very long wiring pattern has to be formed. Thus, the size of the head chip is increased. Also, if the wiring is placed so that a long wiring is inserted in a limited space, edges occur due to change of direction. Thus, if errors of shape occur in the edges, the wiring may be disconnected even at a noise voltage having a small value.
On the other hand, if the material of the fuse member is the same as the material of the heater, resistance of the heater is comparatively high (Rs≅several tens of Ω/□). Thus, the wiring of the fuse member does not need to be formed having a long shape. Part of a vertical structure of a conventional ink-jet printhead having such a fuse array is schematically shown in FIG. 1. Referring to FIG. 1, a fuse array 110 formed of a plurality of fuse members 103, an insulating layer 104, a fuse electrode 105, and a passivation layer 106 are sequentially formed on a base substrate 102 of the ink-jet printhead. A cover member 107 is formed on the passivation layer 106.
In the above structure, the fuse array 110 stores various data by selectively fusing the fuse members 103. Thus, heat is generated by the fuse members 103, and due to the heat, cracks 190 occur in the insulating layer 104 or the passivation layer 106 formed on the fuse members 103. Thus, if ink 108 or external moisture penetrates into the fuse members 103 through the cracks 190, the fuse members 103 may be disconnected or the fuse electrode 105 may be corroded.
FIG. 2 illustrates part of a vertical structure of a ink-jet printhead having a fuse array, which is disclosed in U.S. Pat. No. 6,390,589. Referring to FIG. 2, a fuse array 441 formed of a plurality of fuse members 440 is formed on a base substrate 410, and an insulating layer 450 is deposited on the fuse array 441. A fuse electrode 443 is formed on the insulating layer 450 and is connected to the fuse members 440 via a through hole formed on the insulating layer 450. A passivation layer 452 for insulation is formed on the fuse electrode 443. Also, in order to prevent the passivation layer 452 from being damaged when the fuse members 440 are fused, an anti-cavitation film 453 is formed on the top surface of the passivation layer 452. A cover member including a sealing member 460 and a cover substrate 461 is formed on the top surface of the anti-cavitation film 453.
Here, the fuse members 440 are formed of a material which is the same as the material of a heater for ejecting ink, and a metal layer connected to the heater is used for the fuse electrode 443 such that the fuse array 441 is manufactured without introducing an additional process.
However, since the fuse array 441 is not in contact with the ink, there are limitations in designing a head chip, and due to separation of an ink passage layer which may occur when a printer is used in a bad environment, the ink and external moisture cannot be prevented from penetrating into the fuse member 440.